Tensioning devices of the above-described design are used in different embodiments, preferably in auxiliary unit pulleys of internal combustion engines. Tensioning devices of this type are known both in an internally mounted embodiment, the bearing journal, which is rigidly connected to the tensioning lever, being mounted in the bearing hub which forms part of the base housing, and in an externally mounted embodiment, the bearing hub, which is rigidly connected to the tensioning lever, being mounted on the bearing journal which forms part of the base housing, the base housing being provided in each case for fastening the respective tensioning device to a motor housing, such as for example the crankcase or the control housing of an internal combustion piston engine.
With regard to the arrangement of the tensioning roller, it is additionally possible to distinguish, in a tensioning device of this type, between what is known as an offset or Z embodiment, in which the tensioning roller is arranged axially on the outside of the tensioning lever that is remote from the base housing, and what is known as an in-line embodiment or U embodiment, in which the tensioning roller is arranged radially laterally of the base housing, axially on the inside of the tensioning lever that faces the base housing.
The tensioning lever is mounted radially in or on the base housing via at least one plain bearing sleeve which is arranged between the bearing hub and the bearing journal and is usually made of a durable and at the same time low-friction plastics material. A resulting radial force, which is obtained from the spring force of the torsional spring, acting on the tensioning lever and the reaction force exerted on the tensioning lever by the traction mechanism via the tensioning roller, acts on the bearing sleeve. However, as at least one of the radial planes in which the spring force of the torsional spring and the reaction force of the traction mechanism act on the tensioning lever is usually axially spaced apart from a central radial bearing plane of the pivot bearing or the plain bearing sleeve, there is inevitably obtained a resulting tilting moment about a tilt axis lying perpendicularly to the axis of rotation of the pivot bearing in the central bearing plane. This tilting moment disadvantageously causes non-uniform, one-sided loading, i.e. loading acting axially at the end side in diagonal opposition, of the pivot bearing with high local compressive and edge loading of the plain bearing sleeve, leading to non-uniform wear of the plain bearing sleeve and accordingly to undesirable misalignment of the tensioning lever and also of the tensioning roller fastened thereto with respect to be traction mechanism.
In order to avoid these known drawbacks, various solutions have been proposed to avoid a tilting moment of this type.
Thus, DE 42 20 879 A1 describes a tensioning device with an externally mounted tensioning lever which can be loaded via a torsional spring, which is configured as a helical spring which can be loaded in the closing direction and has end-side spring legs, relative to the base housing with a torsional moment about the axis of rotation of the pivot bearing. On the lever-side outer coil, the helical spring is connected to a sliding block which, in an angular position, which is parallel to a resulting reaction force of a traction mechanism onto the tensioning roller, with respect to the axis of rotation of the pivot bearing, is radially movably guided in a radial guide of an inner cylinder web of the tensioning lever and is pressed by a radial spring force with an inner friction surface against the cylindrical outer wall of an inner cylinder web which is connected to the base housing and arranged coaxially within the inner cylinder web of the tensioning lever.
The radial spring force and thus the frictional moment, which acts between the tensioning lever and the base housing via the friction surface of the sliding block and by which a pivoting movement of the tensioning lever is dampened, behave substantially proportionally to the torsional moment of the helical spring. Furthermore, the radial spring force compensates, via its axial spacing from a central radial bearing plane of the pivot bearing, for the tilting moment of the resulting reaction force of the traction mechanism onto the tensioning roller about a notional tilt axis lying in the central bearing plane of the pivot bearing.
Owing to the relatively small radius of the outer cylinder wall, which is in frictional contact with the sliding block, of the base housing, the frictional moment generated by the radial spring force is comparatively small, or the radial spring force must be relatively large to generate a sufficiently large frictional moment. Furthermore, it is difficult to adjust the radial spring force for exact compensation for the tilting moment of the resulting reaction force of the traction mechanism about the tilt axis. In addition, the known tensioning device has in the region of the base housing, owing to the two required cylinder webs, large radial dimensions which complicate the arrangement of this tensioning device in a traction mechanism drive.
Another tensioning device with an externally mounted tensioning lever is known from EP 0 780 597 B1 in which the tensioning lever can be loaded via a torsional spring, which is configured as a helical spring which can be loaded in the closing direction and has end-side spring legs, relative to the base housing with a torsional moment about the axis of rotation of the pivot bearing. At the housing-side spring end, the helical spring is connected with the inwardly angled spring leg via a ramp surface to a sliding block which is held in an angular position, parallel to a resulting reaction force of a traction mechanism onto the tensioning roller, with respect to the axis of rotation of the pivot bearing and is pressed by a radial component of the acting spring force with an outer friction surface against the cylindrical inner wall of an outer cylinder web connected to the tensioning lever.
The radial component of the spring force and thus the frictional moment, which acts between the tensioning lever and the base housing via the friction surface of the sliding block and by which a pivoting movement of the tensioning lever is dampened, behave substantially proportionally to the torsional moment of the helical spring.
Furthermore, the radial component of the spring force compensates, via its axial spacing from a central radial bearing plane of the pivot bearing, for the tilting moment of the resulting reaction force of the traction mechanism onto the tensioning roller about a notional tilt axis lying in the central bearing plane of the pivot bearing.
Owing to the disadvantageous lever ratios between the spring leg and the sliding block, the frictional moment generated by the radial component of the spring force is comparatively small, or the spring force must be relatively large to generate a sufficiently large frictional moment. Also, it is difficult to adjust the radial component of the spring force for precise compensation for the tilting moment of the resulting reaction force of the traction mechanism about the tilt axis. In addition, the production and assembly costs are relatively high owing to the complex construction of the sliding block arrangement. Furthermore, a spring of this type requires a disadvantageously larger overall space.
A tensioning, device with a sliding block arrangement similar to the aforementioned tensioning device is described in DE 601 05 759 T2. In contrast to the embodiment according to EP 0 780 597 B1, in the embodiment according to DE 601 05 759 T2 the tensioning lever is inwardly mounted and the sliding block arranged at the lever side, the sliding block being formed from a damping plate provided with an outer friction surface or with a friction lining. The helical spring is connected at the lever-side spring end with an inwardly angled spring leg via two contact points to the damping plate, the friction surface of which is arranged in an angular position, parallel to a resulting reaction force of a traction mechanism onto the tensioning roller, with respect to the axis of rotation of the pivot bearing, and is pressed by a radial component of the acting spring force against the cylindrical inner wall of an outer cylinder web connected to the base housing.